Each platform requires UI work to make it really fit the platform in question.
With Xamarin Forms this has changed - you can add a Xamarin Form component and this will work adapt itself across the different platforms. The key word there is adapt - it is not like an HTML page, a Xamarin form input will look different on Android and iOS by default.
So I think your point, while valid, does not apply in any way to Xamarin!
I'm afraid GPs point is still valid - UX (the experience) is more than just native/native-looking widgets, but how the app conforms to platform conventions. For example, iOS apps typically include(d?) a back button at the top left corner; even if the button is rendered faithfully as an Android widget - that app will feel alien on Android (it was also an easy way to spot lazy iOS 'ports'). I think edge-swipes are another iOS convention for navigating back/forward, which would be foreign on Android, so I fully agree with GPs statement: Each platform requires UI work to make it really fit the platform in question
I'll believe it when I see it. Cross-platform UI libraries like Qt, Gtk, SWT, etc, just never end up choosing quite the right widgets or having exactly the right look-and-feel as natively-designed apps.
If cross-platform is a bigger requirement than producing the best-possible UX, then perhaps Xamarin could deliver. I see this being a big deal with enterprise software. But that's almost never the case for most consumer-facing software; you're always much, much better off building your presentation code in a native UI toolkit.
most business users would accept that their apps, look slighly ... unlike common apps on the platform